JP 2002-238290A discloses a motor control unit, which includes an H-bridge circuit, a current detection part having current detection resistors and a control part for feedback-controlling switches forming the H-bridge circuit so that a detection value of the current detection part follows a target value. The H-bridge circuit includes a high-side arm circuit and a low-side arm circuit. The high-side arm circuit is formed of two high-side arm switches connected in parallel to a power supply. The low-side arm circuit is formed of two low-side arm switches connected in parallel to a ground. One of the high-side arm switches and one of the low-side arm switches are connected in series at a first connection node. The other of the high-side arm switches and the other of the low-side arm switches are connected in series at a second connection node. The first connection node and the second connection node are connected electrically to a first terminal and a second terminal of a DC motor, respectively.
The control part alternately performs, as a feedback control, a normal control and a current circulation control. In the normal control, one of the high-side arm switches and one of the low-side arm switches are turned on at the same time so that a current flows from the power supply to the ground through the DC motor. In the current circulation control, one of all high-side switches and all low-side switches are turned on at the same time so that a current, which results from energy stored in the DC motor in the normal control time, flows in the DC motor in the same direction as in the normal control time.
In the conventional motor control unit described above, the current detection resistor is provided at only one side of the DC motor. It is assumed for example that the current detection resistor is provided at a location, which is a high potential side of the motor, that is, a power supply side, in the normal control under normal rotation of the DC motor. In a case that all the high-side arm switches are turned on at the same time in the current circulation control, potential references of the current detection resistor in the normal control and the current circulation control are the ground and the power supply, respectively, under reverse rotation of the DC motor. In a case that all the low-side arm switches are turned on at the same time in the current circulation control, potential references of the current detection resistor in the normal control and the current circulation control are the power supply and the ground, respectively, under normal rotation of the DC motor.
Since a power supply level varies, ranges of error in the detection value of the current detection resistor differ between cases that the references are the power supply and the ground. That is, accuracy of current detection varies. It is not possible to perform the feedback control accurately when the potential references of the current detection resistor differ between the normal control and the current circulation control under the same direction of rotation of the DC motor.